Vehicles such as wheel type loaders include work implements capable of being moved through a number of positions during a work cycle. Such implements typically include buckets, forks, and other material handling apparatus. The typical work cycle associated with a bucket includes sequentially positioning the bucket and associated lift arm in a digging position for filling the bucket with material, a carrying position, a raised position, and a dumping position for removing material from the bucket.
Control levers are mounted at the operator's station and are connected to a hydraulic circuit for moving the bucket and/or lift arms. The operator must manually move the control levers to open and close hydraulic valves that direct pressurized fluid to hydraulic cylinders which in turn cause the implement to move. For example, when the lift arms are to be raised, the operator moves the control lever associated with the lift arm hydraulic circuit to a position at which a hydraulic valve causes pressurized fluid to flow to the head end of a lift cylinder thus causing the lift arms to rise. When the control lever returns to a neutral position, the hydraulic valve closes and pressurized fluid no longer flows to the lift cylinder.
In normal operation, the implement is often brought to an abrupt stop after performing a given work cycle function. This can occur, for example, when the implement is moved to the end of its range of motion. If the lift arms or hydraulic cylinders impact with a mechanical stop, significant forces are absorbed by the lift arm assembly and the hydraulic circuit. This results in increased maintenance and accelerated failure of associated parts.
A similar situation occurs when a control system holds the control lever in a detent position at which the associated hydraulic valve is held open until the lift arm assembly or implement reaches a predetermined position. The control system then releases the control lever which is spring biased to the neutral position. The springs quickly move the control lever to the neutral position which in turn abruptly closes the associated hydraulic valve. Thus, the lift arm assembly and/or bucket is brought to an abrupt stop. Such abrupt stops result in stresses being exerted on the hydraulic cylinders and implement linkage from the inertia of the bucket, lift arm assembly, and load. The abrupt stops also reduce operator comfort and increase operator fatigue.
Stresses are also produced when the vehicle is lowering a load and the operator quickly closes the associated hydraulic valve. The inertia of the load and implement exerts forces on the lift arm assembly and hydraulic system when the associated hydraulic valve is quickly closed and the motion of the lift arms is abruptly stopped. Such stops cause increased wear on the vehicle and reduce operator comfort. In some situations, the rear of the tractor can even be raised off the ground.
To reduce these stresses, systems have been developed to more slowly and smoothly stop the motion of the implement in these situations. One solution to this problem is disclosed in U.S. Pat. No. 4,109,812, issued to Adams et al. on Aug. 29, 1978. A device is provided for halting the flow of hydraulic fluid to the cylinders just prior to the lift arms reaching the end of their range of motion and trapping fluid within the cylinder to act as a hydraulic cushion. While this approach is acceptable for slowing the implement before it reaches a mechanical stop, this device is not readily adapted to use with a control system that stops the implement at adjustable kickout positions. Such kickout positions are chosen in response to the parameters of the work cycle and are typically different from the maximum raise and lower positions. Such a hydraulic cushion is also not readily controllable in response to changes in operating conditions.
An alternative system is disclosed in U.S. Pat. No. 4,358,989, issued to Tordenmalm on Nov. 16, 1982. This system utilizes an electrohydraulic valve to extend and retract a piston within a hydraulic cylinder. When the piston reaches a position that is a predetermined distance from the end of stroke, the control system progressively closes the electrohydraulic valve as the piston continues to move toward the end of stroke. While this system adequately reduces the velocity of the piston before it reaches a hard stop, it is not operable to perform other desirable implement functions, such as adjusting kickout positions and defining multiple raise kickout positions. Also, if the electronic system fails, the operator is unable to operate the hydraulic cylinders.
The present invention is directed to overcoming one or more of the problems set forth above.